STATE-OF-THE-ART IN COUNTERPOISE THEORY

This review, covering the period 1987-1993, deals with quantum chemica l studies that employ the supermolecular method with some form of coun terpoise to calculate the interaction energies of molecular complexes. A theorem is provided showing that the standard counterpoise approach , in which, at a given geometry, energies are evaluated using the full basis set of the complex, yields a pure (i.e. BSSE-free) interaction energy. Artifacts resulting from secondary BSSE have been found to bec ome negligible before the basis set limit of this interaction energy i s attained. Alternative methods in which counterpoise is achieved by r estricting the fragment description in the calculation of the:full com plex to the level used for the isolated fragment are formally nearly c orrect but have been found to give less satisfactory results, mainly b ecause the relevant fragment properties converge only slowly to the ba sis set limit. Some practical aspects are considered as well, such as the ambiguities that arise in calculating the binding energy of a clus ter of molecules.